The electronic properties of a transistor made in an integrated circuit depend on the shape of the transistor and on the materials used to make the transistor. The speed and the threshold voltage of a transistor may vary with the thickness of the gate insulator and/or with the dielectric constant of the material forming the gate insulator. However, depending on the function of the transistor in the integrated circuit, it may be necessary to obtain specific electronic properties, and thus a specific shape or composition of the transistor. For instance, low leakage transistors are known to exhibit low leakage current, and are used for usual analog and logic functions; interface transistors are known to carry high voltages (about 2 volt) and are used for analog functions; and high speed transistors are known to exhibit short response time and are used, for instance, for radio-frequency analog circuits or for high speed analog circuits. The differences of electronic properties of the above-mentioned transistors may be chosen by the features (thickness and/or dielectric constant) of the gate insulator and/or by the features (length or width) of the gate.
Thus, when an integrated circuit comprises several transistors with specific features, it is necessary to provide different fabrication methods. It is then possible to make the different kinds of transistors independently. However, such a process increases the number of processing steps, and thus the cost of the process. Moreover, it is not always possible to obtain the gate insulators with the structural differences (length or width of the gate, thickness or dielectric constant of the gate insulator) allowing to exhibit the desired properties.